Molecular alignment orientation

The results presented in this chapter show that the use of proper effective models, in combination with calculations based on the exact vibrational Hamiltonian, constitutes a promising approach to study the laser driven vibrational dynamics of polyatomic molecules. In this context, the MCTDH method is an invaluable tool as it allows to compute the laser driven dynamics of polyatomic molecules with a high accuracy. However, our models still contain simplifications that prevent a direct comparison of our results with potential experiments. First, the rotational motion of the molecule was not explicitly described in the present work. The inclusion of the rotation in the description of the dynamics of the molecule is expected to be important in several ways. First, even at low energies, the inclusion of the rotational structure would result in a more complicated system with different selection rules. In addition, the orientation of the molecule with respect to the laser field polarization would make the control less efficient because of the rotational averaging of the laser-molecule interaction and the possible existence of competing processes. On the other hand, the combination of the laser control of the molecular alignment/orientation with the vibrational control proposed in this work could allow for a more complete control of the dynamics of the molecule. A second simplification of our models concerns the initial state chosen for the simulations. We have considered a molecule in a localized coherent superposition of vibrational eigenstates but we have not studied the preparation of this state. We note here that a control scheme for the localiza-... 

Nucleation can occur either homogeneously or heterogeneously. Homogeneous nucleation occurs when random molecular motion in the molten state results in the alignment of a sufficient number of chain segments to form a stable ordered phase, known as a nucleus. The minimum number of unit cells required to form a stable nucleus decreases as the temperature falls. Thus, the rate of nucleation increases as the temperature of the polymer decreases. The rate of homogeneous nucleation also increases as molecular orientation in the molten polymer increases. This is because the entropy difference between the molten and crystalline states diminishes as molecular alignment in the molten state increases. 

An amorphous polymer in a state of molecular alignment is not a stable structure - it is metastable. It can uansition either to a more perfectly ordered, crystalline structure, or to a more disordered, nonoriented structure In either case, the free energy of the system is reduced. Given enough time and/or thermal energy, an oriented amorphous polymer will transition in either or both of these directions. 

The molecular theory of Doi [63,166] has been successfully applied to the description of many nonlinear rheological phenomena in PLCs. This theory assumes an un-textured monodomain and describes the molecular scale orientation of rigid rod molecules subject to the combined influence of hydrodynamic and Brownian torques, along with a potential of interaction (a Maier-Saupe potential is used) to account for the tendency for nematic alignment of the molecules. This theory is able to predict shear thinning viscosity, as well as predictions of the Leslie viscosity coefficients used in the LE theory. The original calculations by Doi for this model employed a preaveraging approximation that was later... 

K. Hongladarom, W. R. Burghardt, S. G. Baek, S. Cementwala, and J. J. Magda, Molecular alignment of polymer liquid crystals in shear flows. I. Spectroscopic birefringence technique, steady-state orientation, and normal stress behavior in poly(benzyl glutamate) solutions, Macromolecules, 26, 772 (1993). 

PES is strongly dependent on the molecular bond orientation, there can even be an ntj dependence of diffraction. Miura et al. [74] have shown that this leads to a difference in the rotational alignment of molecules scattered on or off-specular. However, as for rotational inelasticity, there are problems in the comparison of theoretical and experimental diffraction probabilities. 

I /I )2 should be an increasing function of L. One has to keep in mind that due to a two-dimensional orientational distribution function of the in-plane molecular alignment, the contribution of / normalized to the total intensity / +ij, a=/ /(/ +/j ) differs from one even for a homogeneously oriented device, as for example a = 0.75 with S — 3. It is clear that the experimentally observed EL anisotropy plotted against the distance, x, from the A1 cathode (located at x = 0), and identified with varying thickness (L) of parallel oriented n monolayers, contains the EL emission profile determined by the spatial distribution of emitting states, x(x) ... 

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